Communications platforms include towers, balloons, Unmanned Aerial Vehicles (particularly High Altitude Platforms (“HAPS”) and High Altitude Long Endurance (“HALE”) platforms), and satellites at low (“LEO”), medium (“MEO”) and geostationary (“GEO”) Earth orbits. These platforms use directive antennas to form spot beams to provide communication coverage over a specified surface area on Earth referred to as cells. As discussed herein, a cell is a geographical coverage area on the surface of the Earth or in the atmosphere between a communication platform and the area on the surface of the Earth. A spot beam is a radiation pattern of an antenna that illuminates a cell. A surface spectral density (Hertz per square kilometer (“Hz/km2”) within the coverage area is typically increased by increasing the number of radiated spot beams to partition the coverage area into multiple cells and reusing the available spectrum many times. For instance, dividing an area previously covered by one broad beam into 19 cells covered by 19 narrow spot beams and splitting the frequency spectrum into four equal parts (and reusing the spectrum in smaller cells) may result in a surface spectral density that is increased by a factor of 19/4 or nearly five-times. To provide broad and uniform coverage with a high surface spectral density, the telecommunications platform accordingly may use a plurality of antennas such that each antenna is configured to provide similar communication coverage (e.g., a spot beam) to a cell. It is also common practice in satellites to create multiple beams from a single antenna by using more than one duplex feed for each antenna.
In hub-and-spoke networks, also called star networks, communication platforms facilitate communication between at least one gateway station or feederlink station and a plurality of user terminals within a coverage area. The gateway stations are directly connected to the Internet or other local, wide, or geographic computer/television network and configured to provide an Internet/television connection to the user terminals. The communications platforms have at least one feeder link to the gateway station, which is located in one of the cells. The communications platforms convert the forward feeder link uplink signals into forward user link downlink signals provided by the spot beams to user terminals. Similarly, the communications platforms convert user terminal return uplink signals into a return downlink to the gateway station.
Some communications platforms perform demodulation and decoding on each signal received via the feeder link from the gateway station. The signal may include an array of individual baseband packets or time-divided data, which is individually switched by the communications platforms to the appropriate downlink. The communications platforms combine all of the individual baseband packets (or time-divided data) destined for a particular spot beam into a downlink signal, which is coded and modulated for transmission via the spot beam. Some communications platforms also perform the demodulation, decoding, and switching of individual baseband packets (or time-divided data) received in the user terminal return transmissions for transmission to one or more gateway stations, other communications platforms or even other user terminals.
In most communications platforms, however the feeder link with the gateway station uses the same modulation/demodulation and coding/decoding scheme as the links for the user spot beams. Such a configuration is typically optimized for the user terminals because these devices have very low antenna gains and are more frequently subject to fading and noise from environmental factors. This optimization typically includes using modulation and coding schemes that produce relatively more robust links at the expense of being spectrally inefficient, where spectral efficiency is the data rate achieved per unit of spectrum (bps/Hz). If the platform-user terminal link conditions deteriorate due to, for example, heavy rain, an even more robust modulation and coding scheme must be used, thereby further reducing overall system capacity not only on the platform-user terminal link but also on the unimpaired feeder link.